fix(extension/placeholder): Resolve placeholder performance issues

[C-Hess]

Fix #3360

The issue seems to be caused by this line in placeholder.ts:

tiptap/packages/extension-placeholder/src/placeholder.ts

Line 54 in 2b69f34

if (this.editor.isEmpty) {

isEmpty is NOT a simple operation, but uses JSON stringify on the entire document to verify emptiness compared to the default of the given node type.

This wouldn't be too bad on it's own, but it's called for every descendent, causing noticeable performance issues if more than a few empty paragraphs exist on a large document (even on the Moby Dick large text example).

The issue was actually fixed with #2665 but was then reverted by 1024892 for an unknown reason

Credit to @BrianHung for the more efficient empty editor check

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